There exists a need in industry to improve the adhesion and elevated temperature performance of products (e.g., specialty tapes, hook and loop mechanical fasteners, and self-mating mechanical fasteners) to low surface energy (LSE) substrates. "Low surface energy substrate" refers to materials that generally have a surface energy of less than about 45 mJ/m.sup.2, typically less than 40 mJ/m.sup.2, or more typically less than 35 mJ/m.sup.2. Examples of LSE substrates include some powder coatings and polyolefin polymers such as polypropylene (PP), low density polyethylene (LDPE), high density polyethylene (HDPE), and ultrahigh molecular weight polyethylene (UHMWPE). The relative low cost, improved properties, and increased usage of these polyolefin polymers has increased this need. The general rule for adhesives is that there is a tradeoff between peel adhesion and static shear performance. It is especially difficult to achieve the combination of high peel adhesion at room temperature and high static shear performance at elevated temperature. Designing a pressure sensitive adhesive (PSA) to have adhesion to LSE substrates has always been a challenge. Significant development efforts have concentrated on this problem. The challenge has been to develop products and/or systems which have high, consistent, and immediate adhesion to LSE substrates and have high temperature (.gtoreq.70.degree. C. (158.degree. F.)) static shear performance.
Acrylic adhesives can be formulated to have high temperature static shear performance, but generally have poor peel adhesion to LSE substrates. Tackified acrylic adhesives, in particular non-polar acrylates, such as those described in U.S. Pat. No. 5,638,798 (Bennett et al.), have good adhesion to some LSE substrates. However, these adhesives do not typically exhibit desired elevated temperature static shear performance, have difficulty adhering to HDPE, and have a noticeable acrylate odor.
Block copolymer adhesives can be designed to have good adhesion to LSE substrates and have low odor, such as those described in U.S. Pat. No. 5,453,319 (Gobran), but generally block copolymer adhesives have poor elevated temperature static shear performance. The general rule for adhesives stated above also applies to block copolymer adhesives, i.e., the higher the peel adhesion at room temperature, the lower the static shear performance at elevated temperatures.
U.S. Pat. No. 5,028,646 (Miller et al.) describes block copolymer pressure sensitive adhesive containing tackifier resins and preferably 2-20% by weight of an aromatic, essentially hydrocarbon, end block reinforcing resin, which generally has a glass transition temperature (Tg) higher than the service temperature of the adhesive, for refastenable diaper tape applications.
The addition of PPO to block copolymer adhesives to improve the temperature performance is known. For example, U.S. Pat. Nos. 4,104,323 (Hansen) and 4,141,876 (Hansen) disclose the addition of polyphenylene ether (also known as polyphenylene oxide ("PPO")) resin, having a viscosity average molecular weight (M.sub.vis) of between about 6,000 and 25,000 Daltons (Da) and a glass transition temperature of between 170.degree. C. and 200.degree. C., to a block copolymer PSA to provide hot melt adhesive compositions and adhesive tapes having improved service temperature performance. However, these patents do not mention adhesion to low surface energy substrates.
WO 90/14396 (Audett) describes pressure sensitive and hot melt adhesive compositions having improved shear adhesion failure temperatures (SAFT) comprising a block copolymer having at least two monoalkenyl arene polymer endblocks (A) and at least one elastomeric conjugated-diene mid-block (B), said blocks (A) comprising 8-55% by weight of the block copolymer, about 50-200 phr mid-block tackifying resin, and 5-50 phr low molecular weight PPO polymer. The molecular weight (M.sub.vis) of the PPO polymer is from about 1,000 to about 5,000 Da and the Tg is from about 100.degree. C. to 165.degree. C., preferably between 140 and 163.degree. C. This application also generically describes a tape construction utilizing the adhesive composition. Utility of the adhesive for bonding LSE substrates is not specifically mentioned, but T-peel adhesion at room temperature of polyethylene (PE) to itself with the adhesive is reported as 0.0175 kN/m (0.1 lb/inch).
WO 90/14397 (Audett) describes improved shear adhesion failure temperatures (SAFT) adhesive compositions containing low molecular weight PPO similar to that of
WO 90/14396. This application also generically describes tape constructions utilizing these adhesive compositions and substrates coated with these adhesive compositions. T-peel adhesion of 0.0175 kN/m (0.1 lb/in) for PE to PE is exemplified for one adhesive formulation.
WO 97/11997 (Chu) describes use of a polyphenylene oxide delivery system for increasing an upper service temperature of an A-B-A block copolymer adhesive composition. The delivery system comprises a preblend of about 5 to 45 parts by weight of a PPO resin having a Tg with the range of from about 150 to about 210.degree. C. and from about 1 to 450 parts by weight of a B-block (i.e., mid block) compatible resin. This application also describes an adhesive formulation produced with the PPO/resin delivery system. The amount of PPO resin incorporated into the block copolymer adhesive ranges from about 6 to 30% by weight of the total elastomer weight in the formulation. This application also generically describes a tape construction utilizing this adhesive composition. Adhesion to LSE substrates is not mentioned.
U.S. Pat. No. 5,412,032 (Hansen) describes linear styrene-isoprene-styrene block copolymers with an overall molecular weight of above 280,000 up to 520,000 with lower coupling efficiencies to produce adhesive compositions which adhere strongly to difficult to adhere substances such as skin or polyolefins, for example, polyethylene.
It is also known that the use of block copolymers having an increasing diblock content in PSA formulations can improve tack and peel adhesion but generally at the expense of shear performance.
There still exists a need for improved adhesive compositions that bond to LSE substrates having high room temperature peel adhesion and high temperature static shear performance.